Therefore, despite its success in many domains, \(\chi \) EFT remains challenged by nucleon spin observables,
the latest data coming from
dedicated low \(Q^2\)  experiments. Low \(Q^2\)  sum rule measurements are undeniably challenging:
forward angle detection is difficult to reach and subjected to large backgrounds,
a large \(\nu \)  range is needed,
there are low-\(x\)  extrapolations,
avoiding high-\(x\)  contamination requires a careful analysis...
An additional challenge for neutron data is that nuclear corrections are needed and, while the general
agreement between the neutron data coming from deuteron and \(^3\) He is encouraging, one may ask
how reliable the corrections are at low \(Q^2\) .
Yet, the experiments – old and new – provide consistent results and conclusion while being independent and having quite different detectors
and methods.
One must note the disagreement between the state-of-the-art \(\chi \) EFT predictions. But
it does not necessarily indicate an inconsistency. Rather, the difference in \(Q^2\) -behaviors seems to mostly arise from
including [1]} or not [2]}
phenomenological estimates of higher order terms of the \(\chi \) EFT series. In the case where the value of the
observable at \(Q^2=0\)  is not known (e.g. for \(\gamma _0\)  or \(\delta _\mathrm {LT}\) ), an additional important difference
comes from enforcing [1]} or not [2]} consistent couplings.
Therefore, it remains unclear what the origin of the experiment/theory discrepancy is.
A possibility to advance further, even if no new
experiments measuring these observables are foreseen and calculating the next order of the \(\chi \) EFT series
is very difficult, is to compute the observables with other non-perturbative approaches, e.g.
that based on the Dyson-Schwinger Equations, Lattice QCD, Gauge-Duality (AdS/QCD) or global
phenomenological analyses like MAID or SAID [5]}.
It is important to resolve this issue: it challenges our search for a description of Nature at all level since
\(\chi \) EFT is the leading approach to manage the first level of complexity arising above
the strong force sector of the Standard Model.
